Lamp driver circuit

ABSTRACT

An inverter or converter circuit is described for controlling the power to a suitable load such as a gas or vapor discharge lamp. The circuit includes a transformer having a secondary winding coupled to the load and a primary winding connected to a direct-current voltage source by a controlled rectifier such as a silicon-controlled rectifier. A variable-frequency trigger circuit, e.g., a relaxation oscillator, controls the switching rate of the controlled rectifier and the magnitude of the power delivered to the load. An inductive reactance independent of the transformer is connected in series with the primary winding. A capacitive reactance is connected in parallel with the inductive reactance and primary winding to form a resonant tank circuit for providing commutation of the switching device independently of the load impedance.

United States Patent {72] Inventor Clifford D. Skirvin Pomona, Calif.

[2|] Appl. No. 830,958

[22] Filed June 6, 1969 [45] Patented Sept. 28, 1971 [7 3] Assignee Radiant Industries, Inc. North Hollywood, Calif.

[54] LAMP DRIVER CIRCUIT 21 Claims, 3 Drawing Figs.

52 u.s.c| 315/238, 315/100 U, 315/276, 3l5/283 [51 1 Int. Cl H05b 37 00 {50] Field of Search 3l5/l00 U, 100 D, 219, 200, 223, 238, 244, 276, 283, 289,

[56] References Cited UNITED STATES PATENTS 2,756,365 7/1956 Germeshausen 315/289 X 3,466,500 9/l969 Peek 315/100 Primary Examiner-Roy Lake Assistant ExaminerLawrence .l. Dahl ABSTRACT: An inverter or converter circuit is described for controlling the power to a suitable load such as a gas or vapor discharge lamp. The circuit includes a transfomier having a secondary winding coupled to the load and a primary winding connected to a direct-current voltage source by a controlled rectifier such as a silicon-controlled rectifier. A variablefrequency trigger circuit, e.g., a relaxation oscillator, controls the switching rate of the controlled rectifier and the magnitude of the power delivered to the load. An inductive reactance independent of the transformer is connected in series with the primary winding. A capacitive reactance is connccted in parallel with the inductive reactance and primary winding to form a resonant tank circuit for providing commutation of the switching device independently of the load impcdance.

LAMP DRIVER cmcun BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to lamp driver circuits and more particularly to such circuits employing controlled rectifiers.

2. Description of the Prior Art Gas or vapor discharge lamps such as fluorescent, mercury, argon-mercury, mercury-zeon and zeon lamps exhibit a very high resistance characteristic until the gas or vapor within the lamp envelope is ionized. After ionization, such lamps exhibit a negative resistance characteristic. The driver circuits for such lamps must therefore provide a high initial voltage for ignition, e.g., from [1A to 5 times the normal lamp operating voltage. Such lamp driver circuits must also limit the steady state lamp current to not only a safe value but a value which will optimize the light output and the life of the lamp.

Conventional techniques for controlling the current to gas or vapor discharge lamps include a ballast which may utilize either a reactance or resistance in series with the lamp to limit the steady state current. In some cases expensive auxiliary circuits are necessary in addition to-the ballast to provide a sufficiently high starting voltage.

The prior art ballasts consume considerable power thereby reducing the overall lumens per watt output of the system. Normally, such ballasts use an indicative reactance, e. g., in the form of av high-reactance transformer to limit the normal operating current. The use of an inductive reactance as a current-limiting device results in a low-power factor for the ballast. Low-power factor ballasts draw more current from the power supply than ballasts with unity or high-power fact'or type ballast (with same wattage output) and therefore require larger supply conductors. The use of a resistance to limit the lamp current results in an increased power consumption within the ballast. Regardless of the type of current control, conventional ballasts are large, heavy, expensive and very inefficient. Conventional ballasts must also be designed for a specific input voltage and a particular type of vapor or gas lamp.

Electronic converters and inverters have also been used for driving gas or vapor discharge lamps in certain applications. Such electronic devices, although more compact and efficient than conventional ballasts have generally required sophisticated protective circuits to accommodate the wide range of load impedances presented by gaseous discharge lamps. Furthermore, such prior art inverters and converters have proven to be reliable only when used with low input voltages, i.e., less than I volts.

High voltages, i.e., greater than I00 volts, may be readily switched by employing controlled rectifiers of the gate controlled, reverse blocking thyristor type. Controlled rectifiers include a pair of current-carrying electrodes (collector or anode and cathode) and a control or gate electrode. The

rectifiers are turned on by the application of a suitable control voltage to the gate electrode and are turned off (commutated) when the voltage across the current-carrying electrodes decreases below a predetermined level.

Prior art inverter or converter circuits employing such switching devices are generally designed for self commutation by resonating the load or an inductor-capacitor (tank) circuit connected in series or parallel with the load. In either case commutation is dependent upon the magnitude of the load impedance. For this reason controlled rectifiers have found little application in circuits for driving variable impedance loads such as gas or vapor discharge lamps. Controlled rectifiers have been used satisfactorily in gaseous discharge lamp driver circuits only where circuit breakers have been provided to protect the rectifiers from decommutation. Such circuit breakers must have a fast response time to prevent damage to the rectifiers and are therefore expensive.

The above enumerated disadvantages of the prior art lamp driver circuits have been overcome by the present invention.

5 DC within a wide voltage range.

SUMMARY OF THE INVENTION The lamp driver circuit of the present invention includes a pair of input terminals adapted to be connected to a source of DC voltage or rectified AC voltage. A transformer is provided with a primary and secondary winding. A variable impedance load such as a gas or vapor discharge lamp is connected across the secondary winding of the transformer. An inductive reactance independent of the transformer is connected to the primary winding. A capacitive reactance is connected across the primary winding and the inductive reactance for providing a resonant tank circuit. A controlled rectifier including a pair of current-carrying electrodes and a gate electrode is provided for periodically connecting the primary winding of the transformer to the source. The current-carrying electrodes are connected in series with the tank circuit and the input terminals. A trigger pulse generator is coupled to the gate electrode for periodically applying a control signal thereto for rendering the rectifier conductive. The tank circuit is arranged to apply an inverse voltage across the current carrying electrodes of the rectifier after the application of a control signal to provide commutation.

The present invention is described in more detail in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of a preferred embodiment of this invention;

FIG. 2 is a chart showing the waveforms of the voltages preset at various points in the circuit of FIG. I; and

FIG. 3 is a schematic diagram of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, a lamp driver circuit in the form of an inverter includes a pair of input terminals 10, and 12 which are connected to a suitable source of DC voltage such as battery 14. If desired, the input terminals may be connected to an AC source through a suitable rectifier to form a converter.

A transformer 16 includes a primary winding 18 connected in series with the input terminal 10 via a diode 20 and a switch 15. The diode 20 protects the circuit from an improper polarity connection to the battery. The transformer 16 includes a secondary winding 22 having a pair of output terminals 24 and 26 across which a suitable load such as a gas or vapor discharge lamp 28 is connected. The secondary winding 22 includes suitable intermediate taps 30 and 32 for supplying filament current to the lamp electrodes 34 and 36, as illustrated. The transformer 16 is preferably provided with a saturable core to minimize internal losses at low load impedance levels.

A controlled rectifier 40, for example, of the silicon-controlled type (SCR) includes a pair of current carrying electrodes 42 and 44 (i.e., anode and cathode respectively) and a gate electrode 46. The anode 42 is connected to the primary winding 18 of the transformer through an inductive reactance 48. The cathode 44 is connected to the other input terminal 12. Trigger pulse generating means in the form of a relaxation oscillator 50 is coupled to the gate electrode 46 for applying trigger pulses thereto to periodically render the SCR conducting. The relaxation oscillator employs a unijunction transistor 52 having a first base electrode connected to the anode of the SCR by resistors R1 and R2. A second base electrode of the unijunction transistor is connected to the SCR cathode 44 via a bias resistor R3. The emitter of the unijunction transistor is connected to the junction of a potentiometer R4 and a charging capacitor C1. The movable tap of the potentiometer R4 is connected to the junction of the resistors R1 and R2.

The relaxation oscillator operates in a conventional manner. The capacitor Cl charges through R1 and R4 until the emitter voltage reaches a predetermined value at which time the unijunction transistor turns on and discharges Cl through resistor R3. The discharge current through resistor R3 applies a trigger pulse or control signal to the gate electrode 46. The trigger pulse renders the SCR conductors since the voltage across the anode 42 and cathode 44 is positive and above the predetermined tum on voltage. The unijunction transistor 52 ceases to conduct when the emitter voltages reaches a predetermined value. The SCR 40 continues to conduct as a result of the regenerative action therein until the anode voltage 42 drops below a predetermined level or goes negative.

A capacitor 53 is connected across the primary winding 50 and the inductor 48 to form a resonant tank circuit which applies a voltage across the anode 42 and cathode 44 which is below the predetermined turnoff voltage for SCR 40 at a predetermined time after the SCR is turned on by the application of the trigger pulse to the gate electrode 46. The values of the inductor 48 and capacitor 53 are chosen to insure selfcommutation or turnoff of the SCR 40 regardless of the magnitude of the load across the transformer secondary winding 22.

The frequency of operation and the power delivered to the load 28 is determined by the values of capacitor C1 and resistors R1 and R4, the higher the frequency the greater the load current. This frequency, which is preferably set within the range of 5 to 25 kHz. may be adjusted by varying the position of the tap on the potentiometer R4. The potentiometer R4 may be set at the desired operating current level for the lamp 28 or the potentiometer may be used as a convenient dimming control.

By way of example only, the following component values may be used in the driver circuit of FIG. I for supplying current to a 40-watt, l20-volt fluorescent lamp with an input voltage from 60 to 250 volts.

Transformer primary I60 turns on a saturable winding l8 ferrite core Transformer secondary 300 turns winding 22 Inductor 48 50 turns on a sintcrcd iron nonsaturahle core Capacitor 53 l f.

Cl 0.02 pf.

SCR 40 C-22BC UJT 52 2N2646 Referring now to FIG. 2, the voltage waveforms taken at certain points in the circuit of FIG. 1 are illustrated.

Curve A represents voltage across the anode and cathode of the SCR 40. Curve B represents the voltage across the tank circuit or between terminals 54, 56. Curve C illustrates the voltage waveform across the inductor 48. Curve D represents the voltage across the charging capacitor C1. Curve E represents the trigger pulses delivered to the gate electrode 46 by the relaxation oscillator 50 and Curve F represents the voltage delivered to the load or lamp 28. When switch is closed, the voltage across the SCR increases and then begins to decrease (due to the tank circuit) until at time t, the controlled rectifier 40 is rendered conductive. The voltage across the SCR drops to zero and continues into the negative region due to the flyback action of the tank circuit. The negative voltage commutates or turns off the SCR and the cycle repeats it self.

A lamp driver'in the form of a converter is illustrated in FIG. 3 wherein like elements are given the same reference numerals as in FIG. 1. The converter includes a full wave rectifier 60 which may be connected to any suitable source of AC power, for example, i 10 or 220 volts AC for operating a plurality of fluorescent lamps 62, 64 and 66 connected in parallel across the transformer secondary winding 22 as illustrated. The secondary winding in this embodiment need be provided with only one intermediate tap for providing filament current to electrodes on only one end of the lamps. The lamps 62, 64, and 66 are connected in series with ballast capacitors 63, 65, and 67, respectively, to provide a sufficient voltage across the secondary winding 22 to start one or more of the lamps with the remaining lamps in the nonnal operating mode.

in the embodiment of FIG. 3, the trigger pulse generating means comprises a variable resistor R5 and a charging capacitor C2 connected between the padding resistor R1 and the cathode of the SCR. A trigger diode 70 is connected between the junction of resistor R5 and capacitor C2 and the SCR gate electrode 46. The capacitor C2 is periodically charged by the resistor R5 and discharges into the gate electrode 46 via the trigger diode 70, in a well-known manner.

There has thus been described a lamp driver circuit which is capable of operating over a wide range of point voltages. The driver circuit may function as an inverter or a converter. It is inexpensive to manufacture and fail safe. The circuit will continue to function with little internal consumption of power with the output terminals open or short circuited. Various modifications of the driver circuit will be apparent to those skilled in the art without involving a departure from the spirit and scope of my invention.

What is claimed is:

1. In a lamp driver circuit, the combination which comprises:

a pair of input terminals adapted to be connected to a voltage source;

a gaseous discharge lamp;

a transformer having primary and a secondary winding;

means connecting the lamp across the secondary winding of the transformer;

a controlled rectifier having a pair of current-carrying electrodes and a gate electrode, the rectifier being rendered conducting in response to the application of a control signal to the gate electrode and the application of a voltage above a first predetermined value across the currentcarrying electrodes and rendered nonconducting when the voltage across the current carrying electrodes decreases below a second predetermined value;

an inductive reactance independent of the transformer;

means connecting the primary winding and the inductive reactance in series;

a capacitive reactance connected across the primary winding and the inductive reactance for providing a tank circuit;

means connecting the tank circuit and the current-carrying electrodes of the switching means in series across the input terminals; and

trigger-pulse-generating means for periodically generating and applying the control signal to the control terminal of the switching means to render the switching means conducting, the tank circuit being arranged to apply a voltage below the third predetermined value across the current carrying electrodes of the switching means after the application of the control signal to render the switching means nonconducting.

2. The combination as defined in claim 1 wherein the trigger-pulse-generating means includes means for selectively adjusting the repetition rate of the control signals.

3. The combination as defined in claim 2 wherein the switching means is a semiconductor device.

4. The combination as defined in claim 3 wherein the semiconductor device includes an anode and a cathode and wherein the inductive reactance is connected to the anode.

5. The combination as defined in claim 4 wherein the semiconductor device is a silicon-controlled rectifier.

6. The combination as defined in claim 5 wherein the trigger pulse generating means comprises a relaxation oscillator.

7. The combination as defined in claim 6 wherein the transformer includes a saturable core.

8. The combination as defined in claim 6 wherein the relaxation oscillator is arranged to provide a repetition rate within the range of 5 to 25 kHz.

9. The combination as defined in claim 8 wherein the relaxation oscillator includes a unijunction transistor.

10. The combination as defined in claim 8 including a source of direct current voltage connected to the input terminals.

11. In an inverter, the combination which comprises:

a pair of input terminals adapted to be connected to a direct-current voltage source;

a pair of output terminals adapted to be connected to a variable impedance load;

a transformer having a primary and a secondary winding;

means connecting the secondary winding across the output terminals;

a controlled rectifier having an anode, a cathode and a gate;

an inductor independent of the transformer means connecting the primary winding and the inductor in series;

a capacitor connected across the primary winding and the inductor for providing a resonant tank circuit means connecting the tank circuit and the anode and cathode of the controlled rectifier in series circuit relationship across the input terminals; and

trigger-pulse-generating means coupled to the gate of the controlled rectifier for periodically rendering the rectifier conducting, the tank circuit being arranged to apply an inverse voltage across the anode and cathode of the controlled rectifier to turn off the rectifier a predetermined time after the rectifier is rendered conducting, independently of the magnitude of the impedance across the output.

12. The combination as defined in claim 11 wherein the tank circuit is connected to the anode of the controlled rectif- 13. The combination as defined in claim 12 including means for controlling repetition rate of the trigger-pulse-generating means.

14. The combination as defined in claim 13 including a gas or vapor discharge lamp connected across the output terminals.

15. The combination as defined in claim 14 wherein the trigger-pulse-generating means is a relaxation oscillator.

16. The combination as defined in claim 15 wherein the repetition rate is within the range of 5 to 25 kHz.

17. The combination as defined in claim 16 wherein the transformer includes a saturable core.

18. The combination as defined in claim 17 wherein the controlled rectifier is a silicon controlled rectifier.

19. The combination as defined in claim 16 or 18 wherein the inductor is connected to the anode of the controlled rectifier.

20. The combination as defined in claim 19 wherein the relaxation oscillator includes a unijunction transistor; bias means connecting a first base of the transistor to the anode and a second base of the transistor to the cathode of the controlled rectifier, means including a capacitor connecting the emitter of the transistor across the controlled rectifier, and means connecting the second base of the transistor to the gate of the controlled rectifier.

21. The combination as defined in claim 19 wherein the relaxation oscillator includes a trigger diode connected to the gate of the controlled rectifier. 

1. In a lamp driver circuit, the combination which comprises: a pair of input terminals adapted to be connected to a voltage source; a gaseous discharge lamp; a transformer having primary and a secondary winding; means connecting the lamp across the secondary winding of the transformer; a controlled rectifier having a pair of current-carrying electrodes and a gate electrode, the rectifier being rendered conducting in response to the application of a control signal to the gate electrode and the application of a voltage above a first predetermined value across the current-carrying electrodes and rendered nonconducting when the voltage across the current carrying electrodes decreases below a second predetermined value; an inductive reactance independent of the transformer; means connecting the primary winding and the inductive reactance in series; a capacitive reactance connected across the primary winding and the inductive reactance for providing a tank circuit; means connecting the tank Circuit and the current-carrying electrodes of the switching means in series across the input terminals; and trigger-pulse-generating means for periodically generating and applying the control signal to the control terminal of the switching means to render the switching means conducting, the tank circuit being arranged to apply a voltage below the third predetermined value across the current carrying electrodes of the switching means after the application of the control signal to render the switching means nonconducting.
 2. The combination as defined in claim 1 wherein the trigger-pulse-generating means includes means for selectively adjusting the repetition rate of the control signals.
 3. The combination as defined in claim 2 wherein the switching means is a semiconductor device.
 4. The combination as defined in claim 3 wherein the semiconductor device includes an anode and a cathode and wherein the inductive reactance is connected to the anode.
 5. The combination as defined in claim 4 wherein the semiconductor device is a silicon-controlled rectifier.
 6. The combination as defined in claim 5 wherein the trigger pulse generating means comprises a relaxation oscillator.
 7. The combination as defined in claim 6 wherein the transformer includes a saturable core.
 8. The combination as defined in claim 6 wherein the relaxation oscillator is arranged to provide a repetition rate within the range of 5 to 25 kHz.
 9. The combination as defined in claim 8 wherein the relaxation oscillator includes a unijunction transistor.
 10. The combination as defined in claim 8 including a source of direct current voltage connected to the input terminals.
 11. In an inverter, the combination which comprises: a pair of input terminals adapted to be connected to a direct-current voltage source; a pair of output terminals adapted to be connected to a variable impedance load; a transformer having a primary and a secondary winding; means connecting the secondary winding across the output terminals; a controlled rectifier having an anode, a cathode and a gate; an inductor independent of the transformer means connecting the primary winding and the inductor in series; a capacitor connected across the primary winding and the inductor for providing a resonant tank circuit means connecting the tank circuit and the anode and cathode of the controlled rectifier in series circuit relationship across the input terminals; and trigger-pulse-generating means coupled to the gate of the controlled rectifier for periodically rendering the rectifier conducting, the tank circuit being arranged to apply an inverse voltage across the anode and cathode of the controlled rectifier to turn off the rectifier a predetermined time after the rectifier is rendered conducting, independently of the magnitude of the impedance across the output.
 12. The combination as defined in claim 11 wherein the tank circuit is connected to the anode of the controlled rectifier.
 13. The combination as defined in claim 12 including means for controlling repetition rate of the trigger-pulse-generating means.
 14. The combination as defined in claim 13 including a gas or vapor discharge lamp connected across the output terminals.
 15. The combination as defined in claim 14 wherein the trigger-pulse-generating means is a relaxation oscillator.
 16. The combination as defined in claim 15 wherein the repetition rate is within the range of 5 to 25 kHz.
 17. The combination as defined in claim 16 wherein the transformer includes a saturable core.
 18. The combination as defined in claim 17 wherein the controlled rectifier is a silicon controlled rectifier.
 19. The combination as defined in claim 16 or 18 wherein the inductor is connected to the anode of the controlled rectifier.
 20. The combination as defined in claim 19 wherein the relaxation oscillator includes a unijunction transistor; bias means connecting a first base of the transistor to the anode and a second base of the transistor to the cathode of the controlled rectifier, means including a capacitor connecting the emitter of the transistor across the controlled rectifier, and means connecting the second base of the transistor to the gate of the controlled rectifier.
 21. The combination as defined in claim 19 wherein the relaxation oscillator includes a trigger diode connected to the gate of the controlled rectifier. 